1. Field of the Invention
The invention concerns a method and an apparatus for automatically focusing an optical device such as, for example, a surgical stereomicroscope, with which it is possible to focus on the specimen plane which requires the greatest visual acuity.
2. Description of the Related Art
In many applications, especially clinical ones, work is performed in only a small region of a field of view. Moreover, the image content in the small environment in which work is being performed exhibits the greatest changes as a result of movement of the instrument on which the work is being performed. Therefore, as a viewer needs the greatest focal sharpness at precisely where work is being performed, focusing should be performed specifically in this region.
In addition to manual adjustment of the focal plane of an optical device, there are many automatic systems that perform a focal adjustment based on a measurement signal which, in turn, is generated substantially based on a measurement of the distance to the specimen. If the X and Y coordinate axes lie in a plane common to specimen plane, and the Z coordinate axis extends parallel to the optical axis (and perpendicular to the X and Y axes), the X and Y coordinates must be defined in order to ascertain a distance from the optical system to a point in the specimen plane. In simple systems, these coordinates are defined as X0=Y0=0. In more complex systems, the coordinates to be defined (X0, Y0) are designated by means of a joystick. Complex systems can ascertain, for example by “eye tracking,” the coordinates (X0, Y0) of a point being viewed by the observer's eye. In addition, the complex measurement system can ascertain the vertical spacing between that point (X0, Y0) and the objective, i.e. Z0. DE-A-1 928 432, Commissariat à l'Energie Atomique, Paris, which is incorporated herein by reference, describes adapting the Z0 coordinate, based on the change in brightness of the image rays (as the focus setting).
In addition, German Documents DE-A-2 053 017 and DE-B-2 102 922, both of which are incorporated herein by reference, respectively, describe: (a) compensating for a slit image using displaceable mirrors; and (b) an invisible ray bundle with a photoelectric device.
The inventor has recognized that the known systems are disadvantageous in the following respects: (a) a manual or electric focusing system is slow, and diverts the viewer from his or her task or activity; (b) the existing autofocus systems focus on a specific preselected region (X0, Y0) which must be deliberately modified (with manual or electric activation); (c) “eye tracking” devices are: (i) technically complex; (ii) require additional equipment to track the user's eye; and (iii) limit the field of view of assistants, who are observing via a reflected beam path, to the direction of view of the principal observer; and (d) all known methods ignore the use of instruments by the user, or in some circumstances are interfered with thereby.
For these and other reasons, a new apparatus and method are needed for automatically focusing a target region of a specimen.